The invention relates generally to floating structures. More specifically, the invention is directed to a floating structure for supporting a deck structure or other superstructure above a water surface.
Offshore petroleum operations, such as exploration, drilling production, and storage, generally require a deck structure or other superstructure supported above the water surface with sufficient air gap to remain clear of the waves. A superstructure may comprise a diverse array of equipment and structures depending upon the type of offshore operation to be performed. For example, a superstructure for drilling a well and producing hydrocarbons may include equipment for drilling and producing hydrocarbons, living quarters for a crew, equipment storage, and a myriad of other structures, systems, and equipment. During operation, additional payload of drill pipes, drill mud, hydrocarbons, helicopters, and other items may be added. The combined weight of such superstructures and payload is typically measured in thousands of tons. The superstructure may be supported on a generally rigid structure fixed to the seafloor or on a floating structure. Fixed structures are typically viable in shallow waters, typically waters with depths less than 1,000 feet. Floating structures are generally viable in both shallow and deep waters.
There are several basic requirements for a floating structure employed to support a superstructure. The floating structure must provide sufficient buoyancy to support the weight of the superstructure and any payload. The floating structure must be stable in any condition while supporting the weight of the superstructure and payload above the water surface. The floating structure must be able to xe2x80x9ckeep stationxe2x80x9d about a fixed position within a limited range of lateral excursions throughout the duration of a given operation. The floating structure must have acceptable xe2x80x9cseakeepingxe2x80x9d characteristics relating to the oscillatory motions, velocities, and accelerations of the floating structure. The station keeping and seakeeping characteristic requirements are generally determined by operational concerns, such as crew comfort, equipment operability, riser safety, and station keeping system fatigue.
Floating structures generally provide buoyancy through means of a submerged hull employing Archimedes principle. Typically, a void portion of a hull extends below the water surface, displacing a volume of water to provide an uplifting force. Hull construction is typically reinforced steel plating, but other materials, most notably concrete, are also employed. The submerged portion of the hull is most commonly placed directly adjacent to the water surface, such as for a typical ship. Unlike a ship, however, placement of buoyancy is variable.
Floating structures are generally stabilized by one or more of several methods. The first and most common method provides stability through placement of buoyancy directly adjacent to the water surface to create waterplane area. Many configurations of waterplane area are utilized to stabilize the floating structure. Ships are one example wherein a single large waterplane area provides the required stability. A semi-submersible provides an example wherein multiple waterplane areas, spaced widely apart, are employed to reduce the size of the waterplane area required to provide stability. In both examples, as the floating structure pitches and rolls, the center of buoyancy of the submerged hull moves as the waterplane changes to provide a righting moment. While the center of gravity for the floating structure may be located above the center of buoyancy, the floating structure can nonetheless remain stable. Increasing the waterplane area or using multiple, widely spaced waterplanes is generally the cheapest and simplest method for providing stability. The seakeeping consequences of a large waterplane, however, are generally undesirable.
The second method provides stability by placement of the center of gravity of the floating structure below the center of buoyancy. The combined weight of the superstructure, hull, payload, ballast and other elements may be arranged to be below the center of buoyancy. The floating structure will pitch about the center of rotation with the reversed pendulum effect of the weight providing a righting force. Arrangement of the center of gravity below the center of buoyancy may be a difficult task. One method employed to lower the center of gravity requires the addition of fixed ballast below the center of buoyancy to counterbalance the weight of superstructure and payload. Fixed ballast, generally is a negatively buoyant hull structure or material added to the floating structure to lower the center of gravity. There are two main types of fixed ballast, structural weight and non-structural solid ballast. Examples of structural fixed ballast include permanent ballast tanks, flooded truss portions, and concrete oil storage tanks. Examples of solid ballast include metal filings, pig iron, iron ore, and concrete placed within or attached to the hull structure. The advantage of the weight arrangement is that it may be achieved such that seakeeping performance is unaffected while stability is increased. Another method is to move the center of buoyancy higher, generally by placing buoyancy adjacent to or near the water surface. The disadvantage of buoyancy rearrangement is that it may require an increasing waterplane area and a hull structure near the water surface, both generally having negative seakeeping consequences.
The third method provides stability by arrangement of station keeping elements attached between the seafloor and the floating structure. Typically, marine tendon systems are composed of sections of steel pipe arranged vertically. The tendons are attached in a widely dispersed pattern about the center of rotation of the floating structure. Pitching of the floating structure induces elongation in the tendons on one side of the center of rotation and contraction on the other side to produce a righting moment. The pretension on the tendons also act in a manner similar to solid ballast. The pretension functions as ballast weight lowering the effective center of gravity for the floating structure. Tendon-based platforms have heretofore generally been costly floating structures. This result is due to the large tendons required to provide adequate vertical stiffness and pretension along with complications associated with the installation of rigid tendons. The cost of tendon-based floating structures also tends to increase significantly with water depth, due to a reduction in tendon stiffness that occurs as tendon length increases. Tendon size must be increased to maintain the required vertical stiffness, resulting in costs which may geometrically increase with water depth. The advantage is that seakeeping performance for tendon-based structures is generally superior due to the extreme stiffness of a marine tendon system in the vertical, or heave, direction. Floating structures whose vertical stiffness is primarily controlled by the stiffness of attached station keeping elements, rather than the vertical stiffness of the waterplane, shall be referred to as tendon-based floating structures.
Floating structures may employ the aforementioned methods of stabilization, either alone or in combination. Those floating structures whose stability is satisfied upon an arrangement of waterplane area or placement of the centers of gravity and buoyancy may be referred to as self-stabilizing floating structures. Such floating structures have the advantage of being stable independent of the function of an external station keeping system. The seakeeping characteristics of self-stabilizing floating structures not employing tendons, however, is generally inferior to that of tendon-based floating structures employing station keeping elements to provide or augment stability. Marine tendon systems, however, have heretofore generally been seen as unfeasible for ultra deep water operations due to increasing costs and installation difficulties.
A floating structure is generally subject to excursion and motion in six degrees of freedom, as illustrated in FIG. 1. Displacements in the vertical direction, longitudinal, and transverse directions are generally referred to as heave, surge, and sway, respectively. Rotations about the heave, surge, and sway axes are generally referred to as yaw, roll, and pitch, respectively. However, since many offshore oil structures are symmetric in the surge and sway directions, the terms lateral excursion or surge shall be used as inclusive of displacements or motions in either direction. Further, the term tilt or pitch shall be used as inclusive of displacements or motions in either the pitch or roll directions.
A floating structure may also be subject to the environmental forces of wind, waves, and current. The magnitude of these forces is generally controlled by design and arrangement of the hull, superstructure, and other elements of a floating structure. These forces combine to induce the generally undesirable response of steady excursions and oscillatory motions in the aforementioned six degrees of freedom. It is frequently desirable for a floating structure to remain relatively stationary either in relation to a fixed point on the seafloor or relative to another body during an offshore operation. Holding a floating structure upon a fixed mean position, or station, and reducing lateral excursions about this station against the forces of the environment shall be referred to as station keeping.
Station keeping may be provided by a number of means. Short-term operations allow the use of dynamic positioning systems to provide some or all of the station keeping requirements. Dynamic positioning systems generally employ active means of monitoring position combined with thruster control to hold a fixed position. Most applications requiring fixed position operations, however, employ station keeping elements attached between the seafloor and the floating structure. The station keeping elements, typically steel pipe rigid tendons or steel wire and chain mooring lines, fix the mean position. Station keeping elements act directly to reduce the static lateral excursions of the floating structure about the mean position. Station keeping elements, however, are generally not directly effective to reduce dynamic motions. Instead, as previously mentioned, design and arrangement of the elements of the floating structure directly control dynamic motions by determining the magnitude of environmental forces applied to the floating structure. Station keeping elements do, however, have an indirect affect on dynamic motions by altering the natural periods of motion for a given floating structure design. Therefore, a combination of hull and station keeping system design may be employed to determine and reduce the dynamic response of a floating structure under environmental forces. The characteristic dynamic motion response of a floating structure, including any system of attached station keeping elements under environmental forces, shall be referred to as seakeeping.
The seakeeping characteristics of a floating structure are determined by a number of factors, importantly: size of the waterplane, submerged hull profile, and natural periods of motion of the floating structure. Several principles generally apply. As waterplane area increases, wave induced heave forces increase. As the size of the vertical cross-sectional hull shape, or hull profile, in a zone nearest the water surface increases, wave induced surge forces increase. This area near the water surface wherein the majority of the wave-induced hydrodynamic forces occur, shall be referred to as the wave zone. The manipulation and affect of floating structure natural periods of motion is a more complex subject explained in more detail below. In general, however, two principles may be mentioned. As the total mass, including added mass, of the floating structure increases, the natural periods of motion become longer. As the total stiffness of a floating structure against excursion in a particular direction increases, the natural period of motion in that direction decreases.
A floating structure may be modeled as a spring mass system having a natural period of vibration in the heave and surge directions described by the following formula:
Tn=2xcfx80{square root over (M/K)}
where for a given direction:
Tn=Natural Period of the Mooring System
M=Mass of the System including Added Mass
K=Stiffness of the System
In the vertical or heave direction, the stiffness of a floating structure is generally determined by the water plane area of the submerged hull and the vertical stiffness characteristics of any attached tensile attachments, such as mooring lines or tendons. The most common method of increasing vertical stiffness is through the use of a marine tendon system. The hull of the floating structure is submerged, generally such that the total buoyancy provided is in excess of floating structure and payload weight. The additional buoyancy acts as pretension on the tendons. Therefore, the heave motion of the floating structure induces elongation of the tendons. The total vertical stiffness for such a floating structure would be the total of the combined stiffness of all tendons and the stiffness added by the waterplane. The stiffness added by the waterplane, however, is generally small compared with the combined tendon stiffness. A tendon-based floating structure is generally characterized as having a vertical stiffness roughly an order of magnitude or more larger than the vertical stiffness supplied by the waterplane area alone.
The Mass (M) of a floating structure may be defined most simply as the mass of all matter that moves when the floating structure moves. For engineering purposes, Mass (M) has two components: displacement and added mass. Displacement includes all attached and captured mass, comprising attached items such as the superstructure, payload, hull structure, and solid ballast, and captured weight such as ballast water or hydrocarbons held in tanks. Added mass is a more foreign concept, generally including a portion of the water around the hull of the floating structure which is forced to move as the floating structure moves. The amount of added mass may be varied through hull design. Added mass may or may not be desirable depending upon the requirements of a particular floating structure. Added mass, however, is generally the cheapest method of increasing the mass of a floating structure for purposes of influencing the natural period of motion.
When a floating structure is stationed in an open sea environment, the floating structure is exposed to the forces of wind, current, and waves. Wind and current may be generally steady for time scales on the order of a natural period of an offshore structure, therefore generally inducing a non-oscillating, or static, offset with some relatively smaller amounts of slow drift oscillation. Wave patterns, however, are generally irregular on these time scales, and generally induce an offset having both a static portion and an oscillating portion. The oscillating portion comprises both dynamic motions occurring near the wave period and slow drift motions occurring near the natural period of motion of the floating structure.
An irregular wave surface is characterized by the presence of a large number of individual waves with different wave periods and wave heights. The statistical properties of such a surface may be described by means of a wave-energy spectrum or wave energy distribution such as illustrated in FIG. 2(a). The motion response of a floating structure may be characterized by means of a Response Amplitude Operator (RAO) such as illustrated in FIG. 2(b). The expected motion response spectrum of the floating structure may be derived by the product of the wave energy spectrum and the square of the RAO, as illustrated in FIG. 2(c). By way of example, the primary wave period for a one hundred year hurricane condition in the Gulf of Mexico is between fourteen and sixteen seconds. This environmental condition is often used as a design environmental condition for floating structures employed in the Gulf of Mexico. The surge natural period of a typical moored offshore structure employed in the Gulf of Mexico for production operations is on the order of 100-300 seconds. This is due to the relatively small lateral stiffness (K) provided by station keeping elements as compared with the mass (M) of the floating structure. As can be appreciated by reference to FIGS. 2(a) to 2(c), the surge motion response spectrum may be a double peaked curve. The first peak, representing the first order motions occurring near the primary wave period, may be significantly smaller than the second peak, representing the slow drift motions occurring near the surge natural period of the floating structure. A relatively small input of wave energy, generally corresponding to relatively small magnitude environmental forces, may induce large resonant response motions in a degree of freedom having a long natural period of motion, typically surge. In other degrees of freedom, the length of the natural period may be nearer to the primary wave period. Where a natural period of motion and a primary wave period coincide or nearly coincide, a motion amplification phenomenon referred to as resonance matching occurs. Extremely large amplitude motions may result from resonant matching. It is therefore desirable that a floating structure have no natural period of motion in any degree of freedom that falls near the primary wave period.
The vertical stiffness of a floating structure is generally much stiffer than its lateral stiffness. This is due to the stiffness provided by the waterplane, apart from the use of tendons. The result is that resonance matching may occur in heave. Therefore, floating structures are generally designed to have heave natural periods significantly above or below the primary wave period. This factor has divided floating structures into two basic categories. One category, comprises tendon-based floating structures, having heave natural periods (Tn) under the primary wave period, typically near five seconds. The other category, generally comprises non-tendon based, self-stabilizing floating structures, having heave natural periods (Tn) over the primary wave period, generally greater than twenty seconds. By way of example, a typical floating structure employing a marine tendon system, such as a tension leg platform, may have a heave natural period (Tn) of three to five seconds. A floating structure not employing a marine tendon system, such as a spar buoy platform or semi-submersible, generally has a heave natural period (Tn) above twenty seconds.
The result is that prior art tendon-based floating structures are sensitive to Mass (M), as increasing the mass (M) of the floating structure results in an increased tendon requirement. Vertical stiffness (K) must be increased in order to retain a low heave natural period (Tn). Conversely, non-tendon based structures are sensitive to vertical stiffness (K). Tradeoffs must generally be made between stability and seakeeping, as decreasing waterplane area decreases stability while increasing heave natural period (Tn).
Prior art floating structures have been developed which employ a variety of means for providing buoyancy, stability, station keeping, and seakeeping. As a means of illustration of the aforementioned floating structure design concerns, several exemplary floating structures are discussed.
A semi-submersible provides an example of a self-stabilizing floating structure employing an arrangement of waterplane area to provide stability. FIG. 3 illustrates an exemplary semi-submersible 300 comprising a drilling platform 302 positioned on the hull structure 304. The hull structure 304 comprises multiple columns 306 upon submerged pontoons 308 which provide the required buoyancy. The center of gravity (CG) of the semi-submersible 300 is above the center of buoyancy (CB). The required stability is therefore provided by wide spacing between the waterplane area of the columns 306. The relatively large vertical cross-sectional area of the hull, or hull profile, in the wave zone, induces relatively large environmental forces in the lateral direction. A semi-submersible, therefore, has relatively large requirements for a station keeping system. A spread pattern of conventional catenary mooring lines 310 may be employed to perform station keeping. The mooring lines 310 are run through fairleads 312 generally placed near the waterline, extending in a catenary shape to anchors 314 at the seafloor 320. The natural periods of motion in all six degrees of freedom are generally above twenty seconds. The size and spacing of the waterplane, however, result in relatively large heave and pitch seakeeping characteristics. When employed for drilling operations, as illustrated in FIG. 3, a single drilling riser 316 extends between the superstructure 302 and a drilling template 318 on the seafloor 320. The drilling riser 316 may be disconnected during periods of large motions. In production operation, top tensioned steel risers are generally not employed due to the relatively large motions experienced by a semi-submersible. Instead flexible risers are generally used whenever water depth permits. Otherwise, steel catenary risers might be feasible for greater depths.
A spar buoy provides an example of a self-stabilizing floating structure employing an arrangement wherein the center of buoyancy (CB) is above the center of gravity (CG) to provide stability. FIG. 4 illustrates an exemplary spar buoy 400 comprising a drilling and production superstructure 402 positioned on a single columnar hull 404 structure, typically extending more than six hundred feet below the water surface. The relatively large hull profile in the wave zone, induces relatively large environmental forces in the lateral direction. A spar buoy, therefore, has relatively large requirements for a station keeping system. A spread pattern of conventional catenary mooring lines 406 may be employed to perform station keeping. The mooring lines 406 are run through fairleads 408 generally placed near the center of buoyancy (CB), extending in a catenary shape to anchors or piles 410 at the seafloor 320. The size of the waterplane may be relatively small, as a spar buoy does not heavily rely on waterplane area for stability. Despite a small waterplane, the length of the hull 404 must be long enough to provide sufficient fixed ballast mass and added mass such that the heave natural period is more than twenty seconds to provide a relatively small heave seakeeping characteristic. Spar buoys are, however, subject to relatively large pitch seakeeping characteristics, in addition to relatively large lateral excursions. When employed for production and drilling operations, as illustrated in FIG. 4, risers 412 extend between the superstructure 402 and a template 414 on the seafloor 320. Riser weight may be supported by buoyancy tanks (not shown) along the length of the risers 412, in an open well in the center of the hull 404 protected from waves.
A Tension Leg Platform (TLP) provides an example of a floating structure employing a marine tendon system to augment stability. FIG. 5 illustrates an exemplary TLP 500 comprising a drilling and production superstructure 502 positioned on a hull structure 504. The hull structure 504 is a semi-submersible type, comprising multiple columns 506 upon submerged pontoons 508 which provide the required buoyancy. A configuration of rigid tendons 510 is attached between the base of the columns 506 and a tendon template 512 at the seafloor 320. The center of gravity (CG) of a TLP, like other semi-submersibles, is above the center of buoyancy (CB). The required stability may, therefore, be provided by wide spacing between the waterplane area of the columns 506. In a tow condition, a TLP may be self-stabilizing. In operation condition, however, a TLP is generally dependent upon tendons to augment stability. Pretension is applied to the rigid tendons 510 generally in the range of twenty to thirty-five percent of the TLP""s 500 displacement. Pretension increases stability by lowering the effective center of gravity (CG) and greatly increasing the vertical stiffness. This reliance upon tendons to augment stability may, however, result in relatively large tension variations in the rigid tendons 510 during operation. The required vertical stiffness may be on the order of 2,000 tons per foot to provide a heave natural period of three to five seconds. The hull profile in the wave zone is still relatively large, inducing relatively large environmental forces in the lateral direction. Tendons alone may, nonetheless, be sufficient to perform station keeping. Despite having relatively large lateral excursions, unlike a semi-submersible, a TLP generally has very small heave and pitch seakeeping characteristics. When employed for production and drilling operations, as illustrated in FIG. 5, risers 514 extend between the superstructure 502 and a template 516 on the seafloor 320. Riser weight may be supported by conventional hydraulic or hydro-pneumatic tensioners, due to the small motions of a TLP. The performance of the TLP is generally superior to other options. The cost of construction and installation, however, have relegated its usage to large petroleum deposits. Further, the TLP has generally been seen as having a viable depth limit due to the increasing costs and complications associated with using rigid tendons.
A Mini-Tension Leg Platform (Mini-TLP) provides an example of floating structure employing a marine tendon system to provide stability without being self-stabilizing. Mini-TLP designs have been developed in an attempt to take advantage of the performance of a TLP at a lower cost. FIG. 6 illustrates an exemplary Mini-TLP 600 comprising a drilling and production superstructure 602 positioned on a hull structure 604. The hull structure 604 is a single column 606 upon a single submerged pontoon 608 which provides the required buoyancy. Outriggers 610 are attached about the column 606 and pontoon 608. Rigid tendons 612 are attached between the outriggers 610 and a template 614 at the seafloor 320. The center of gravity (CG) of a Mini-TLP, like a full-sized, semi-submersible type TLP, is above the center of buoyancy (CB). The waterplane area, however, may be insufficient to supply any significant amount of stability. Instead, stability is derived almost wholly from the tension in the rigid tendons 612 applied through the lever arm created by the outriggers 610. Again, this may result in relatively large tension variations in the rigid tendons 612 during operation. The size of the waterplane area is smaller than that of a conventional TLP, reducing the environmental forces in the heave direction. The hull profile in the wave zone is also smaller, reducing the environmental forces in the lateral direction. The heave natural period of a Mini-TLP may therefore be allowed to be longer than that of a TLP due to the reduced environmental loading. The heave natural period might be permitted to increase to slightly more than five seconds. Tendons 612 are generally sufficient to perform station keeping. A Mini-TLP is generally not as stable as a full-sized, semi-submersible type TLP. This has the consequence of reducing the allowable superstructure and payload weight to retain acceptable heave and pitch seakeeping characteristics, while simultaneously retaining acceptable tendon tensions. When employed for production and drilling operations, as illustrated in FIG. 6, it has also been claimed that due to the small relative motions between the base of the pontoon 608 and the risers 616, submerged linear spring tensioners 618 may be employed. Otherwise, steel catenary risers (not shown) attached at the base of the pontoon 608 and extending in a catenary shape to the seafloor 320, may be employed. The performance and economy of the Mini-TLP design has been demonstrated. The limitation on superstructure weight, however, has heretofore relegated its usage to relatively small petroleum deposits. Further, the Mini-TLP is also felt to have a viable depth limit due to the increasing costs associated with using rigid tendons.
A Tension Buoyant Tower (TBT) provides an example of a cross-over structure employing a marine tendon system. FIG. 7 illustrates an exemplary TBT 700 essentially comprising a production superstructure 702 having a work-over rig 704 positioned on hull structure 706. The hull structure 706 is basically a truss type spar buoy hull comprising a single column 708 upper portion above a submerged truss 710 portion with a bottom portion 712 filled with solid ballast. The column portion 708 provides the required buoyancy. The truss 710 and bottom portion 712 provide fixed ballast and added mass. One or more rigid tendons 714 are attached between the hull 706 and a template 716 at the seafloor 320. The center of buoyancy (CB) is above the center of gravity (CG), providing the required stability. The waterplane area and rigid tendon(s) 714 further augment stability. The hull profile in the wave zone is similar to that of a spar buoy; however, rigid tendon(s) 714, rather than mooring lines, are employed to perform station keeping. As a result of employing rigid tendon(s) 714, the heave natural period has been disclosed as less than five seconds, while all other natural periods of motion remain above twenty seconds. Like spar buoys, a TBT may be subject to relatively large lateral excursions and pitch seakeeping characteristics. The primary benefit claimed is the reduced complexity of the station keeping system over a conventional spar buoy. The allowable superstructure weight is also generally seen to be limited. The TBT is generally seen as most economical for small field production operations, functioning essentially as a single buoyancy tank used to support the weight of multiple risers. As illustrated in FIG. 7, risers 718 extend between the superstructure 702 and the template 716 at the seafloor 320. Riser weight is supported by the buoyancy of the hull 706.
Two important lessons may be appreciated from the above discussion of prior art structures. It is generally desirable for a floating structure to have minimal waterplane area to reduce wave induced heave and pitch motions and to reduce the magnitude of wave induced tensions in the tendons. It is also generally desirable for a floating structure to have a minimum vertical cross-sectional area, or hull profile, in the wave zone to reduce the magnitude of wave induced lateral excursion and reduce the requirements for station keeping systems. In response to these lessons, prior art floating structures have been developed having both minimal waterplane areas and relatively small hull profiles in the wave zone.
A Mini-Tension Leg Platform (Mini-TLP), such as that illustrated in FIG. 6, provides an example of a minimal waterplane and hull profile floating structure. Other designs have been developed employing truss rather than column structures in the wave zone. A truss structure is generally accepted as the preferred support structure for supporting weight while having minimal wave loading. The truss is the paradigmatic structure used for fixed platforms in shallow water. FIG. 8 illustrates an exemplary Truss Mini-TLP 800 comprising a production superstructure 802 supported by a cross-braced truss support structure 804 above a submerged pontoon 806. The pontoon 806 provides the required buoyancy. Rigid tendons 808 are attached between the outer edge of the pontoon 806 and a template 810 at the seafloor 320. The ceriter of gravity (CG) of a Truss Mini-TLP is generally well above the center of buoyancy (CB). Having virtually no waterplane area, stability is derived almost exclusively from the rigid tendons 110 and the lever arm created by the width of the pontoon 806. The heave natural period of a Truss Mini-TLP is similar to that of other Mini-TLP""s. The profile of the hull in the wave zone is small, greatly reducing the size of environmental forces in the lateral direction. The distance below the water surface at which the pontoon 806 may be placed is, however, limited by the ability of the rigid tendons 808 to offset the decreased stability as the center of buoyancy (CB) and center of gravity (CG) diverge. The allowable weight of the superstructure and payload is likewise limited. When employed for production and drilling operations, as illustrated in FIG. 8, a riser configuration similar to that of other Mini-TLP designs may be employed. The Truss Mini-TLP also is generally felt to have a viable depth limit due to the increasing costs associated with using rigid tendons.
A design known as Floating Jacket provides an example of a non-tendon based, minimal waterplane and hull profile floating structure. FIG. 9 illustrates an exemplary Floating Jacket 900 comprising a production superstructure 902 having a work-over rig 904 supported by a cross-braced truss support structure 906 above a deeply submerged hull structure 908. The hull structure 908 provides the required buoyancy. Mooring lines 910 attached between the hull structure 908 and the seafloor 320 are run through fairleads 912 near the center of buoyancy (CB). The Floating Jacket is a self-stabilizing floating structure. Having virtually no waterplane area, stability is derived from placement of the center of gravity (CG). Given the wide separation of superstructure weight and the center of buoyancy (CB), significant fixed ballast weight is required. A portion of the hull located a distance below the center of buoyancy (CB) is filled with solid ballast such as concrete or other negatively buoyant material. The quantity of solid ballast weight and distance of placement below the center of buoyancy (CB) must be sufficient to counterbalance the superstructure, payload, and other weight above the center of buoyancy (CB). The natural periods of motion in all degrees of freedom is generally much longer than that of other prior art floating structures, most notably, the heave natural period is disclosed as being from eighty-five to over one hundred seconds. The profile of the hull in the wave zone is practically negligible, as the hull structure may be submerged completely beyond the wave zone, greatly reducing the size of environmental forces in the lateral direction. The allowable weight of the superstructure and payload is limited only by the cost of adding additional fixed and solid ballast. The seakeeping characteristics of the Floating Jacket are generally superior to other prior art structures. The Floating Jacket has very small dynamic motion seakeeping characteristics in surge and pitch. Only heave motions are significant, though still much less than that of other floating structures such as a semi-submersible.
While a promising concept, the Floating Jacket did not address three concerns which prevented industry acceptance. First, as a result of the minimal waterplane area, the vertical stiffness of the Floating Jacket is too small to be practical. The vertical stiffness is on the order of several tons per foot, making the Floating Jacket unsuitable for drilling. As a general rule, it is desirable to have a minimum of one hundred tons per foot vertical stiffness to allow drilling operations. Additionally, such a low vertical stiffness may allow severe draft changes when superstructure payload changes are made. A helicopter landing may cause the superstructure to rapidly submerge several feet. Rapid, large amplitude draft changes are generally unacceptable. Rapid draft changes are extremely detrimental to stability in structures dependent upon the reversed pendulum effect for stability. In addition, risers 914 connected between the superstructure 902 and at a template 916 at the seafloor 320 employ tensioning systems (not shown) to prevent riser buckling. The risers 904 must remain in tension at all times during operation. Tensioning systems are most sensitive to draft changes. Rapid, large amplitude draft changes greatly increase riser fatigue and could result in catastrophic riser buckling. Second, while dynamic pitch motions are small, the static pitch angle under strong wind may be excessive. The long distance between the superstructure 902 and center of buoyancy (CB) result in large pitch moments from wind forces on the superstructure. The righting moment to pitch is generally limited to the reversed pendulum effect of the center of gravity (CG) about the center of rotation. Mooring lines 910 provide insignificant righting moments, as they are located near the center of buoyancy (CB). This placement is required due to loop current concerns. Placement of the mooring lines at a location other than the center of buoyancy (CB) would have the mooring lines themselves inducing an overturning moment when the hull is subjected to current. Finally, installation of the Floating Jacket would be difficult and expensive. The Floating Jacket is stable in the installed condition, but stability may be a concern during installation operations; the length of the hull and truss may require their assembly in multiple pieces offshore; and the low vertical stiffness and deep submergence of buoyancy makes setting a heavy superstructure difficult.
As can be appreciated from the foregoing discussion of prior art structures, many attempts have been made to solve a basic conflict between stability and seakeeping where a floating platform is employed to support a superstructure above a water surface. It is convenient and desirable to place buoyancy at or near the water surface for stability reasons. Large waterplane area and hull profile, however, induce undesirable large amplitude wave forces to produce large motions and station keeping system requirements. One solution is to submerge the buoyancy, as the dynamic wave forces decrease exponentially with water depth. As much as three quarters of such hydrodynamic forces occur in the upper one hundred feet nearest the water surface. Prior attempts at floating structures employing submerged buoyancy have encountered various performance limitations. Prior tendon-based floating structures may be subject to depth and superstructure weight limitations generally incident to Mini-TLP configurations. Further, these floating structures may be subject to sensitivity to the addition of superstructure, payload, and hull weight in order to retain a heave natural period of motion below that of the primary wave period. Prior non-tendon based floating structures may be subject to operational limitations related to small vertical stiffness and the lack of available righting moments. Further, these floating structures also may encounter difficulty and high cost in installation.
In general, in one aspect, the invention relates to a floating offshore structure comprising a buoyant hull which contains sufficient fixed ballast to place the center of gravity of the floating structure below the center of buoyancy of the hull. A support structure coupled to an upper end of the hull supports and elevates a superstructure above the water surface. A soft tendon has a first end attached to the hull and a second end attached to the seafloor. A vertical stiffness provided by the soft tendon results in the floating structure having a heave natural period of at least twenty seconds.
In general, in another aspect, the invention relates to a hull for a floating offshore structure comprising a positively buoyant upper portion connected to a negatively buoyant lower portion. The lower portion contains a sufficient amount of fixed ballast to place a center of gravity of the floating offshore structure below a center of buoyancy of the floating offshore structure. At least one soft tendon having a first end attached to the lower portion of the hull and a second end attached to the seafloor, wherein a vertical stiffness provided by the tendon results in the floating offshore structure having a heave natural period of at least twenty seconds.
In general, in another aspect, the invention relates to a station keeping arrangement for a floating offshore structure comprising a buoyant hull which contains sufficient ballast to place a center of gravity of the floating offshore structure below a center of buoyancy of the floating offshore structure. A tendon connector is attached to the hull. At least one soft tendon having a first end attached to the tendon connector and a second end attached to a seafloor provides a vertical stiffness which results in the floating offshore structure having a heave natural period of at least twenty seconds.
In general, in another aspect, the invention relates to a method of installing a floating offshore structure comprising providing a single caisson buoyant hull having a support structure coupled thereto, and towing the hull and support structure in a vertical orientation to a predetermined offshore location, the hull floating on or near a water surface during the towing and providing sufficient waterplane area to maintain stable floatation of the floating offshore structure. The method further comprises adding ballast to the hull to submerge the hull below a water surface such that a center of gravity of the floating offshore structure is below a center of buoyancy of the floating offshore structure.
In general, in another aspect, the invention relates to a method of station keeping for a floating offshore structure including a buoyant hull, a support structure, and a superstructure. The method comprises adding sufficient ballast to the hull to place a center of gravity of the floating offshore structure below a center of buoyancy of the floating offshore structure, and attaching a first end of a soft tendon to the hull and a second end of the tendon to a seafloor, wherein a vertical stiffness provided by the tendon results in the floating offshore structure having a heave natural period of at least twenty seconds.
Other aspects of the invention will be apparent from the following description and the appended claims.